Switching fibers for textiles

ABSTRACT

A method of forming a color-changing fiber that can be incorporated into fabrics and other woven materials. The color changing fibers include an annular wall and a conductive wire axially extending through the annular wall, a core strand surrounded by the annular wall and extending axially through a central portion of the fiber, and an encapsulated electro-optic medium disposed on a surface of the core strand.

RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.17/973,653, filed Oct. 26, 2022 (published as U.S. Patent PublicationNo. 2023/0044551), which is a divisional of U.S. patent application Ser.No. 16/851,175, filed Apr. 17, 2020 (published as Patent Publication No.2020/0241330), which is a continuation-in-part of U.S. patentapplication Ser. No. 16/585,218, filed Sep. 27, 2019 (published as U.S.Patent Publication No. US2020/0103720), which claims priority to U.S.Provisional Patent Application No. 62/739,684, filed Oct. 1, 2018. Thisapplication additionally claims priority to U.S. Provisional PatentApplication No. 62/835,660, filed Apr. 18, 2019. All patents, patentapplications, and references disclosed herein are incorporated byreference in their entireties.

BACKGROUND OF INVENTION

There are many applications for clothing that can change on demand. Ifmodern fabrics were able to change color on demand, a consumer coulddramatically reduce the number of articles of clothing that he or shepurchased in a lifetime. It would no longer be necessary to have, forexample, three different blouses of nearly identical cut but differentcolor. The consumer could simply chose the color (or pattern) neededdepending upon the event, season, etc. In this way, color changingfabrics could greatly reduce the environmental impact of clothing.Additionally, replacing these clothes with each new fashion season isresource-intensive—regardless of the source of the fabric, e.g., cotton,wool, or petrochemicals. Other applications for color changing clothinginclude camouflage and sportswear. For example, a baseball team would nolonger require two different uniforms, the color could be changeddepending upon whether the team was home or away.

A variety of technologies have been identified for creating fabrics thatare able to reversibly change colors. These technologies includethermochromic dyes, which change color when exposed to differenttemperatures, photochromic dyes, which change color when exposed tosunlight, integrated LEDs, which can be illuminated on demand byproviding power to the diodes, and liquid crystal inks, which allowdifferent colors to be shown (or not) with the presence of a suppliedelectric field. These technologies have been highlighted in variousprototypes, but only the thermochromic dyes have been widelyincorporated into clothing. See “Hypercol or” t-shirts sold by GeneraSportswear. However, because the thermochromic clothing is heatsensitive, the color patterns are variable. For example, the underarmsof a t-shirt having thermochromic ink may be consistently a differentshade, drawing attention to that area.

One proposed solution is forming a hollow fiber that is subsequentlyfilled with an electro-optic medium, such as an electro-phoretic medium,as is disclosed in U.S. Patent Application Publication No. 2018/0364518.The fiber may be prepared by using a syringe, for example, to fill anextruded hollow fiber that has conductive wire electrodes imbeddedlengthwise with a liquid electro-optic medium comprisingelectrophoretically active pigment particles dispersed in a non-polarsolvent. However, some of the disadvantages associated with thisproposed fiber include undesired pigment settling, difficulties infilling appreciable lengths of fiber whose hollow cavity dimension isless than 200 microns, and inability to cut the fiber to differentlengths without compromising the function of the entire fiber as aresult of dispersion leaking out of the hollow fiber's internal cavity.

Thus, there still remains a need for inexpensive and robust fabrics thatcan change color on demand.

SUMMARY OF INVENTION

The invention overcomes the shortcomings of the prior art by providingflexible fibers that can be switched between colors on demand that aremore mechanically robust. The fibers may be incorporated into fabrics byweaving, knitting, embroidering, thermoforming, or matting. The fiberscan be incorporated into other materials to achieve strength,breathability, or stretch as demanded by the application. When asuitable electric field is provided, the color of the fiber will switch.Because the pigments are bistable, it is not necessary to provideconstant power to maintain the color state. Rather, once the fabric isswitched, it is stable for long periods of time, e.g., days or weeks.

Accordingly, in one aspect, a flexible color-changing fiber may comprisean annular wall and a conductive wire axially extending through theannular wall, a core strand surrounded by the annular wall and extendingaxially through a central portion of the fiber, and an encapsulatedelectro-optic medium disposed on a surface of the core strand. Theelectro-optic medium may be bistable and comprise a non-polar solventand one or more sets of charged pigment particles.

In another aspect, a method of making a flexible color-changing fibermay comprise coating at least a portion of a surface of a core strandwith an encapsulated electro-optic medium, inserting the coated corestrand through a preform, inserting a conductive wire through thepreform, and drawing down the preform to form a fiber comprising anannular wall, such that the conductive wire extends axially through theannular wall and the core strand extends axially through a centralportion of the fiber,

The creation of fibers containing bistable electronic ink and thesubsequent incorporation of the fibers into fabrics and apparel, etc.,would enable switching of the fabrics and then disconnecting them fromelectronics because the display is stable with no power. Accordingly,the drive electronics would not have to be integrated into the fabricunless mobile switching was desired. Thus, in some embodiments, aswitching box, which could be battery powered, is a detachableaccessory. The lack of driving electronics greatly simplifies launderingthe fibers while also increasing durability. If it is desirable to havethe device changing actively while worn, the switching electronics couldbe included in the garment but would only have to be turned on for briefperiods during the updates.

These and other other aspects of the present invention will be apparentin view of the following description.

BRIEF DESCRIPTION OF DRAWINGS

The drawing Figures depict one or more implementations in accord withthe present concepts, by way of example only, not by way of limitations.In the figures, like reference numerals refer to the same or similarelements,

FIG. 1A is a schematic cross-sectional view of a flexible color-changingfiber according to a first embodiment of the present invention.

FIG. 1B is a schematic cross-sectional view of a flexible color-changingfiber according to a second embodiment of the present invention.

FIG. 2 is a schematic cross-sectional view of a flexible color-changingfiber according to a third embodiment of the present invention.

FIGS. 3A and 3B illustrate a system for making the flexiblecolor-changing fiber of FIG. 2 .

DETAILED DESCRIPTION

In the following detailed description, numerous specific details are setforth by way of examples in order to provide a thorough understanding ofthe relevant teachings. However, it should be apparent to those skilledin the art that the present teachings may be practiced. without suchdetails.

The invention provides flexible color-changing fibers that can beincorporated into textiles and other materials. The ability to includeelectronic components into the fiber, i.e., wire electrodes, is usefulfor attaining a practical and economical fiber based display. Forexample, referring to FIG. 1A, a schematic cross-sectional view of acolor-changing fiber according to first embodiment of the invention isprovided.

As illustrated in FIG. 1A, the color-changing fiber 10 comprises alight-transmissive annular wall having a substantially rectangularcross-section and a coated core strand 12 extending coaxially through acentral portion of the fiber 10. In some embodiments, the fiber has asubstantially rectangular cross-section and the inner core strand has asubstantially circular cross-section. However, other cross sectionalshapes for the annular wall and central core strand are also possible,such as an ovoid. The rectangular cross section may have sharp edges, asshown in FIG. 1A, or the corners may be slightly rounded. The annularwall of the color changing fibers may be made of various polymers, mostpreferably a light-transmissive thermoplastic, Examples of polymersinclude, but are not limited to, polycarbonate, poly(methylmethacrylate), polyethylene terephthalate, amorphous copolyester,polyvinyl chloride, liquid silicone rubber, cyclic olefin copolymers,polyethylene, ionomer resins, polypropylene, fluorinated ethylenepropylene, styrene methyl methacrylate, styrene acrylonitrile resin,polystyrene, and methyl methacrylate acrylonitrile butadiene styrene.

The thickness of the central core strand 12 is selected so that a largeenough outer surface area is provided to facilitate coating the outersurface of the core strand 12 with an electro-optic medium 14, but nottoo large as to result in a stiff fiber that will be difficult forfabric weaving. The core strand 12 is made from a conductive material,preferably one or more metals or other conductive material with goodductility. Exemplary materials include, but are not limited to,tungsten, copper, nickel, aluminum, stainless steel, gold, silver,alloys thereof, and combinations thereof. Alloys of the aforementionedconductive metals may also be incorporated in the core strand. Theconductive metal(s) may also be plated onto the surface of anon-conductive or conductive substrate to form the core strand, forexample. A larger thickness for the central core strand will alsofacilitate aggressive cleaning of the wire in order to expose the wirefor electrical connections to a power source and/or controller, forexample. Preferably, the central core strand has a thickness greaterthan or equal to about 20 microns and less than or equal to about 250microns.

Prior to the application of the electro-optic medium, at least a portionof the outer circumferential surface of the central core strand 12 ispreferably coated with a layer of semi-conductive material 13. The layerof semi-conductive material may be provided in the form of an annularcoating having a thickness from about 5 microns to about 200 microns,preferably to about 50 microns, wherein the thickness is preferablymeasured between the core strand 12 and the electro-optic medium 14. Thesemi-conductive material 13 preferably comprises a doped polymericmaterial. The composition and thickness of the semi-conductive materialis selected, such that the semi-conductive material provides forelectrical driving signals across the full circumferential area of theelectro-optic medium between core strand 12 and the conductive wires 18a, 18 b, described in greater detail below. This phenomenon is alsoknown as “blooming” whereby the area of the electro-optic layer whichchanges optical state in response to a change of voltage is larger thanthe area of the electrode, in this example, the cross-sectional areathrough the diameter of the conductive wires 18 a, 18 b. To promoteblooming, an optional layer of light-transmissive semi-conductivematerial (not shown) may also be applied to at least a portion of theouter circumferential surface of the electro-optic medium 14. The term“light-transmissive” is used herein to mean that the layer thusdesignated transmits sufficient light to enable an observer, lookingthrough that layer, to observe the change in optical states of theelectro-optic medium; in cases where the electro-optic medium displays achange in reflectivity at non-visible wavelengths, the term“light-transmissive” should of course be interpreted to refer totransmission of the relevant non-visible wavelengths. The resistivity ofthe layer of semi-conductive material is preferably about 10 e9 ohm-cmor less, more preferably about 10 e7 ohm-cm or less, at 20° C.

Doped polymeric materials that may be used in the layer ofsemi-conductive material may include, but are not limited to, aliphaticor aromatic polyurethane latexes, polyacrylates, and poly(meth)acrylatescontaining a dopant, such as tetrabutylammonium hexafluorophosphate,1-butyl-3-methylimidazolium hexafluorophosphate, polyvinylalcohol,ionically modified polyvinylalcohol, gelatin, polyvinylpyrrolidone, andcombinations thereof. Polymeric blends containing aromatic isocyanatesare less preferred. Examples of formulations that may be included in thelayer of semi-conductive polymeric material are described in U.S. PatentApplication Publication No. 2017/0088758 and U.S. Pat. Nos. 7,012,735;7,173,752; and 9,777,201. The semi-conductive material is preferablyhydrophilic, preferably water insoluble, so that the semi-conductivematerial is not dissolved or removed during application of theelectro-optic medium, which may be applied as an aqueous slurry to thecore strand.

The layer of electro-optic medium 14 applied over at least a portion ofsurface of the layer of semi-conductive material 13 and the central corestrand 12 is preferably a solid electro-optic material. Someelectro-optic materials are solid in the sense that the materials havesolid external surfaces, although the materials may, and often do, haveinternal liquid- or gas-filled spaces. Thus, the term “solidelectro-optic material” may include rotating bichromal members,encapsulated electrophoretic media, and encapsulated liquid crystalmedia.

Electro-optic media of a rotating bichromal member type are described,for example, in U.S. Pat. Nos. 5,808,783; 5,777,782; 5,760,761;6,054,071 6,055,091; 6,097,531; 6,128,124; 6,137,467; and 6,147,791(although this type of media is often referred to as a “rotatingbichromal ball,” the term “rotating bichromal member” is preferred asmore accurate since in some of the patents mentioned above the rotatingmembers are not spherical). Such media uses a large number of smallbodies (typically spherical or cylindrical) which have two or moresections with differing optical characteristics, and an internal dipole.These bodies are suspended within liquid-filled vacuoles within amatrix, the vacuoles being filled with liquid so that the bodies arefree to rotate. The appearance of the material is changed by applying anelectric field thereto, thus rotating the bodies to various positionsand varying which of the sections of the bodies is seen through aviewing surface. This type of electro-optic medium is typicallybistable.

The terms “bistable” and “bistability” are used herein in theirconventional meaning in the art to refer to electro-optic materialshaving first and second states differing in at least one opticalproperty, and such that after the electro-optic material has beendriven, by means of an addressing pulse of finite duration, to assumeeither its first or second state, after the addressing pulse hasterminated, that state will persist for at least several times, forexample at least four times, the minimum duration of the addressingpulse required to change the state of the electro-optic material. It isshown in U.S. Pat. No. 7,170,670 that some particle-basedelectrophoretic materials capable of gray scale are stable not only intheir extreme black and white states but also in their intermediate graystates, and the same is true of some other types of electro-optic media.This type of media is properly called “multi-stable” rather thanbistable, although for convenience the term “bistable” may be usedherein to cover both bistable and multi-stable media.

The term “gray state” is used herein in its conventional meaning in theimaging art to refer to a state intermediate two extreme optical states,and does not necessarily imply a black-white transition between thesetwo extreme states. For example, several of the E Ink patents andpublished applications referred to below describe electrophoreticmaterial in which the extreme states are white and deep blue, so that anintermediate “gray state” would actually be pale blue. Indeed, asalready mentioned, the change in optical state may not be a color changeat all. The terms “black” and “white” may be used hereinafter to referto the two extreme optical states of a material, and should beunderstood as normally including extreme optical states which are notstrictly black and white, for example the aforementioned white and darkblue states. The term “monochrome” may be used hereinafter to denote adrive scheme which only drives electro-optic media to their two extremeoptical states with no intervening gray states.

Another type of electro-optic media uses an electrochromic medium, forexample an electrochromic medium in the form of a nanochromic filmcomprising an electrode formed at least in part from a semi-conductingmetal oxide and a plurality of dye molecules capable of reversible colorchange attached to the electrode; see, for example O′Regan, B., et al.,Nature 1991, 353, 737; and Wood, D., Information Display, 18(3), 24(March 2002). See also Bach, U., et al., Adv. Mater., 2002, 14(11), 845.Nanochromic films of this type are also described, for example, in U.S.Pat. Nos. 6,301,038; 6,870,657; and 6,950,220. This type of medium isalso typically bistable.

Another type of electro-optic media may be found in electro-wettingdisplays developed by Philips and described in Hayes, R. A., et al.,“Video-Speed Electronic Paper Based on Electrowetting”, Nature, 425,383-385 (2003). It is shown in U.S. Pat. No. 7,420,549 that suchelectro-wetting media can be made bistable.

One type of electro-optic media, which has been the subject of intenseresearch and development for a number of years, is the particle-basedelectrophoretic media, in which a plurality of charged particles movethrough a fluid under the influence of an electric field.Electrophoretic media can have attributes of good brightness andcontrast, wide viewing angles, state bistability, and low powerconsumption when compared with liquid crystal displays.

As noted above, electrophoretic media require the presence of a fluid.In most prior art electrophoretic media, this fluid is a liquid, butelectrophoretic media can be produced using gaseous fluids; see, forexample, Kitamura, T., et al., “Electrical toner movement for electronicpaper-like display”, IDW Japan, 2001, Paper HCS1-1, and Yamaguchi, Y.,et al., “Toner display using insulative particles chargedtriboelectrically”, IDW Japan, 2001, Paper AMD4-4). See also U.S. Pat.Nos. 7,321,459 and 7,236,291.

Numerous patents and applications assigned to or in the names of theMassachusetts Institute of Technology (MIT), E Ink Corporation, E InkCalifornia, LLC, and related companies describe various technologiesused in encapsulated electrophoretic and other electro-optic media.Encapsulated electrophoretic media comprise numerous small capsules,each of which itself comprises an internal phase containingelectrophoretically-mobile particles in a fluid medium, and a capsulewall surrounding the internal phase. Typically, the capsules arethemselves held within a polymeric binder to form a coherent layerpositioned between two electrodes. The technologies described in thesepatents and applications include:

(a) Electrophoretic particles, fluids and fluid additives; see forexample U.S. Pat. Nos. 7,002,728 and 7,679,814;

(b) Capsules, binders and encapsulation processes; see for example U.S.Pat. Nos. 6,922,276 and 7,411,719;

(c) Films and sub-assemblies containing electro-optic materials; see forexample U.S. Pat. Nos. 6,982,178 and 7,839,564;

(d) Backplanes, adhesive layers and other auxiliary layers and methodsused in displays; see for example U.S. Pat. Nos. 7,116,318 and7,535,624;

(e) Color formation and color adjustment; see for example: U.S. Pat.Nos. 7,075,502 and 7,839,564;

(f) Methods for driving displays; see for example U.S. Pat. Nos.7,012,600 and 7,453,445; and

(g) Applications of displays; see for example U.S. Pat. Nos. 7,312,784and 8,009,348.

Many of the aforementioned patents and applications recognize that thewalls surrounding the discrete microcapsules in an encapsulatedelectrophoretic medium could be replaced by a continuous phase, thusproducing a so-called polymer-dispersed electrophoretic display, inwhich the electrophoretic medium comprises a plurality of discretedroplets of an electrophoretic fluid and a continuous phase of apolymeric material, and that the discrete droplets of electrophoreticfluid within such a polymer-dispersed electrophoretic display may beregarded as capsules or microcapsules even though no discrete capsulemembrane is associated with each individual droplet; see for example,the aforementioned U.S. Pat. No. 6,866,760. Accordingly, for purposes ofthe present application, such polymer-dispersed. electrophoretic mediaare regarded as sub-species of encapsulated electrophoretic media.

Encapsulated electrophoretic media typically does not suffer fromclustering and settling failure and provides further advantages, such asthe ability to print or coat the media on a wide variety of flexible andrigid substrates. (Use of the word “printing” is intended to include allforms of printing and coating, including, but without limitation:pre-metered. coatings such as patch die coating, slot or extrusioncoating, slide or cascade coating, curtain coating; roll coating such asknife over roll coating, forward and reverse roll coating; gravurecoating; dip coating; spray coating; meniscus coating; spin coating;brush coating; air knife coating; silk screen printing processes;electrostatic printing processes; thermal printing processes; ink jetprinting processes; electrophoretic deposition (See U.S. Pat. No.7,339,715); and other similar techniques.) Further, because the mediumcan be primed (using a variety of methods), an application utilizing themedium can be made inexpensively.

It is preferred that the electro-optic media used in the variousembodiments of the present invention is provided in the form ofmicroencapsulated electrophoretic media. For example, referring again toFIG. 1A, a layer of electro-optic medium 14 comprising a plurality ofcapsules 15 containing a dispersion of charged pigment particles in asolvent may be coated as an annular coating around the central corestrand 12. The annular coating may have a thickness greater than orequal to about 10 microns, preferably about 15 microns, more preferablyabout 20 microns, and less than or equal to about 250 microns,preferably about 100 microns, more preferably about 75 microns, and mostpreferably about 50 microns. The microcapsule coating may be provided,for example, in the form of an aqueous coating slurry formulationcomprising a microencapsulated dispersion of electrophoretic particlesand a binder. The binder material may include, but is not limited to, anaqueous polymeric latex dispersion or water-soluble polymer solutions(e.g. polyvinyl alcohol, such as Kuraray Poval® CM-318, fish gelatin,and alginate). The slurry formulation may further comprises one or moreadditives, such as hydropropyl methyl cellulose, surfactants (e.g TritonX-100), and co-solvents (e.g. butanol).

The charged pigment particles may be of a variety of colors andcompositions. Additionally, the charged pigment particles may befunctionalized with surface polymers to improve state stability. Suchpigments are described in U.S. Patent Publication No. 2016/0085132,which is incorporated by reference in its entirety. For example, if thecharged particles are of a white color, they may be formed from aninorganic pigment such as TiO₂, ZrO₂, ZnO, Al₂O₃, Sb₂O₃, BaSO₄, PbSO₄ orthe like. They may also be polymer particles with a high refractiveindex (>1.5) and of a certain size (>100 nm) to exhibit a white color,or composite particles engineered to have a desired index of refraction.Black charged particles, they may be formed from CI pigment black 26 or28 or the like (e.g., manganese ferrite black spinel or copper chromiteblack spinel) or carbon black. Other colors (non-white and non-black)may be formed from organic pigments such as CI pigment PR 254, PR122,PR149, PG36, PG58, PG7, PB28, PB15:3, PY83, PY138, PY150, PY155 or PY20.Other examples include Clariant Hostaperm Red D3G 70-EDS, Hostaperm PinkE-EDS, PV fast red D3G, Hostaperm red D3G 70, Hostaperm Blue B2G-EDS,Hostaperm Yellow H4G-EDS, Novoperm Yellow HR-70-EDS, Hostaperm GreenGNX, BASF Irgazine red L 3630, Cinquasia Red L 4100 HD, and Irgazin RedL 3660 HD; Sun Chemical phthalocyanine blue, phthalocyanine green,diarylide yellow or diarylide AAOT yellow. Color particles can also beformed from inorganic pigments, such as CI pigment blue 28, CI pigmentgreen 50, CI pigment yellow 227, and the like. The surface of thecharged particles may be modified by known techniques based on thecharge polarity and charge level of the particles required, as describedin U.S. Pat. Nos. 6,822,782, 7,002,728, 9,366,935, and 9,372,380 as wellas US Publication No. 2014-0011913, the contents of all of which areincorporated herein by reference in their entirety.

The particles may exhibit a native charge, or may be charged explicitlyusing a charge control agent, or may acquire a charge when suspended ina solvent or solvent mixture. Suitable charge control agents are wellknown in the art; they may be polymeric or non-polymeric in nature ormay be ionic or non-ionic. Examples of charge control agent may include,but are not limited to, Solsperse 17000 (active polymeric dispersant),Solsperse 9000 (active polymeric dispersant), OLOA 11000 (succinimideashless dispersant), Unithox 750 (ethoxylates), Span 85 (sorbitantrioleate), Petronate L (sodium sulfonate), Alcolec LV30 (soy lecithin),Petrostep B100 (petroleum sulfonate) or B70 (barium sulfonate), AerosolOT, polyisobutylene derivatives or polyethylene co-butylene)derivatives, and the like. In addition to the suspending fluid andcharged pigment particles, internal phases may include stabilizers,surfactants and charge control agents. A stabilizing material may beadsorbed on the charged pigment particles when they are dispersed in thesolvent. This stabilizing material keeps the particles separated fromone another. As is known in the art, dispersing charged particles(typically a carbon black, as described above) in a solvent of lowdielectric constant may be assisted by the use of a surfactant. Such asurfactant typically comprises a polar “head group” and a non-polar“tail group” that is compatible with or soluble in the solvent. In thepresent invention, it is preferred that the non-polar tail group be asaturated or unsaturated hydrocarbon moiety or another group that issoluble in hydrocarbon solvents, such as for example apoly(dialkylsiloxane). The polar group may be any polar organicfunctionality, including ionic materials such as ammonium, suifonate orphosphonate salts, or acidic or basic groups. Particularly preferredhead groups are carboxylic acid or carboxylate groups. Stabilizerssuitable for use with the invention include polyisobutylene andpolystyrene. In some embodiments, dispersants, such as polyisobutylenesuccinimide and/or sorbitan trioleate, and/or 2-hexvldecanoic acid areadded.

Embedded within the annular wall 11 of the color changing fiber 10 aretwo conductive wires 18 a, 18 b. The conductive wires 18 a, 18 b,similar to the core strand 12, preferably comprise one or more metals orother conductive materials having good ductility that include, but arenot limited to, tungsten, copper, nickel, aluminum, stainless steel,gold silver, alloys thereof, and combinations thereof. Variousembodiments of the present invention may include only a singleconductive wire embedded in the annular wall of the color changingfiber; however, it is preferred that at least two conductive wires, morepreferably at least three conductive wires, are embedded in the annularwall.

The conductive wires 18 a, 18 b and core strand 12 serve as electrodesfor applying an electric field across the encapsulated electro-opticmedium 14. For example, if the charged pigment particles within thecapsules 15 have a positive charge polarity, the pigment particles maybe driven away from the core strand 12 by applying a positive voltage tothe core strand 12 relative to the conductive wires 18 a, 18 b. Thiswould switch the color changing fiber 10 to an optical state associatedwith the color of the pigment particles. The charged pigment particlesalternatively may be driven towards the core strand 12 by applying anegative voltage to the core strand 12 relative to the conductive wires18 a, 18 b. If the pigment particles are dispersed within a coloredsolvent, this would result in switching the optical state of the colorchanging fiber 10 to one associated with the color of the solvent.Alternatively, the dispersions within the capsules 15 may contains twosets of pigments particles having opposite charge polarities anddifferent colors to switch the optical state of the fiber (e.g. whitenegatively charged particles and black positively charged particles).Additionally, it is possible to provide levels of graystates by mixingthe two pigment particles within the capsules.

It is desirable to have the fiber be thin enough for weaving intodevices but thick enough to enable perceptible active switching. It isalso desirable to have the annular wall of the fiber as thin as possibleto maximize the optical switching volume relative to the thickness ofthe fiber. However, if the walls are too thin the fiber will fail,either by cracking, snapping, or by releasing one of the wireelectrodes. Toward that end, it is desirable for a fiber having arectangular cross section to be approximately 2 mm×1 mm thick, or less,with an inner cavity having a diameter of 1 mm, preferably 0.5 mm, orless. In preferred embodiments, the rectangular fiber can be about 0.8mm×0.5 mm or less and the diameter of the internal cavity about 0.4 mmor less.

As previously noted, flexible fibers made according to some of theembodiments of the present invention include one or more conductive wireelectrodes, such as conductive wires 18 a, 18 b in FIG. 1A, runninglengthwise along the fiber as close to the electro-optic medium aspossible without compromising the ability of the annular wall tomechanically restrain the electrodes. The electrodes may be formed fromtungsten, silver, copper, or other conductive material with goodductility. The electrode cross-sectional shape may be round, rectangularor other shape that will optimize the uniformity of the electric fieldacross the electro-optic medium. When the ends of the electrodes areconnected to an electrical supply, an electric potential can be createdacross the electro-optic medium to cause a change in its optical state.The electrodes should be as small as possible while maintaining enoughmechanical strength to survive the fiber making process. In some of theembodiments of this invention, the electrodes are typically 100 μm orless in diameter and preferably 50 μm or less in diameter. The fiberscan be indefinitely long, for example, 1 meter or longer, e.g., 10meters or longer, e.g., 100 meters or longer.

Referring now to FIG. 1B, a cross-sectional schematic view of a colorchanging fiber 16 according to a less preferred embodiment of thepresent invention is illustrated. The color changing fiber 16 comprisesmany of the same features as the first embodiment. The color changingfiber 16 comprises an annular wall 11 that may be made from a polymericmaterial, such as polycarbonate, and a central portion occupied by acore strand 17 having a circumferential coating of a semi-conductivematerial 13 and an encapsulated electro-optic medium 14. Again, anoptional second coating of semi-conductive material (not shown) may beapplied to the outer surface of the electro-optic medium 14. The colorchanging fiber 16 of FIG. 1B differs from the embodiment illustrated inFIG. 1A in that the core strand 17 is non-conductive and additionalconductive wires 18 a, 18 b, 18 c, 18 d are embedded in the annular wall11 for applying an electric field to the electro-optic medium 14. Thecore strand 17 may be a synthetic fiber comprising a non-conductivepolymer and functions as a substrate for coating the layers ofsemi-conductive material 13 and electro-optic medium 14. Examples ofsynthetic materials for the core strand include, but are not limited to,cellulosic fibers (e.g. regenerated cellulose and cellulose triacetate),polyamides (e.g. polycaprolactam and polyhexamethylene adipamide),aramids (e.g. poly-p-phenylene tereph-thalamide, poly-m-phenyleneisoph-thalamide), polyesters (e.g. polyethylene terephthalate),polyolefins (e.g. polyacrylonitrile, polypropylene, polyethylene), andpolyurethane.

The electric field applied to the electro-optic medium 14 may beachieved by applying a voltage between pairs of conductive wires. Forexample, the top pair of conductive wires 18 a, 18 b may have the samepolarity, but opposite to the polarity of the bottom pair of conductivewires 18 c, 18 d. Therefore, the conductive wires 18 a, 18 b, 18 c, 18d, may be made from one or more conductive materials, such as theconductive materials previously described. Because the color changingfiber 16 has a rectangular cross-section, it is beneficial that thecross-section is wider than deep when viewed from one side (e.g. the topor bottom side of the rectangular cross-section illustration of FIG.1B), This creates a better ratio of active area to the inactive area,i.e. area not obscured by the conductive wires, while keeping the gapbetween the electrode wires smaller, thereby achieving a more homogenouselectric field. Typically the ratio of width to height in cross sectionis of at least 1.2:1, more preferably 1.5:1, or more, when using arectangular cross-section. The aspect ratio is also important forcontrolling the orientation of the fiber in products that incorporatethe fiber (e.g., fabrics). For example, a fiber with a rectangularcross-section will consistently bend in one direction without rolling,allowing the fiber to be oriented so that it shows a certain face. Thiseffect is important when the fiber includes encapsulated electrophoreticmedia having more than one optical state resulting in the opposite sidesof the fiber having different colors.

It is preferred that the embodiment of FIG. 1B include four conductiveelectrode wires arranged in two sets of two wires. The wires arepreferably arranged close to the electro-optic medium and toward themiddle of the height of the fiber for improved uniformity of theelectric field generated by the wires. The fraction of the widthelectro-optic medium that is obscured by opaque electrode wires is givenby the formula f=N*D/W, where N is the number of wires on one side ofthe electro-optic medium, D is the wire diameter and W is the diameterof the central portion comprising the electro-optic medium. It isdesirable for the fraction f to be less than 40% and preferably lessthan 20%.

Referring now to FIG. 2 , a color changing fiber 20 according to yetanother embodiment of the present invention is illustrated. The colorchanging fiber of the third embodiment is essentially the same as thefirst embodiment except that the color changing fiber 20 has a circularcross-section. The color changing fiber 20 comprises an annular wall 21having conductive wires 28 a, 28 b extending axially therethrough, aswell as a core strand 22 that is also conductive extending co-axiallythrough a central portion of the fiber 20. The outer surface of the corestrand 22 is coated with a semi-conductive material 23 and anencapsulated electro-optic medium 24. The same materials previouslydescribed for the annular wall 21, conductive wires 28 a, 28 b, corestrand 22, semi-conductive material 23, and electro-optic medium 24 maybe used. Again, an optional layer of light-transmissive semiconductivematerial (not shown) may be applied to the outer circumferential surfaceof the electro-optic medium 24.

A system and method of making a color changing fiber according to thevarious embodiments of the present invention is illustrated in FIGS. 3Aand 3B. The system comprises a heater or furnace 32 having an internalbore into which a cylindrical preform 30 is loaded. The furnace 32 isillustrated with a section removed to reveal the cylindrical preform 30within the bore. The preform 30 is not limited to a cylindrical shapehaving a circular cross-section, but may, for example, be provided inthe form a block having a square or rectangular cross-section or acylinder with an oval cross-section. The preform 30 also includes acentral cavity extending co-axially through the length of the preform30, as well as one or more additional cavities extending axially throughthe annular wall of the preform 30. A conductive wire is inserted intoeach of the cavities in the annular wall of the preform 30, such asconductive wires 33 a, 33 b.

A coated core strand 34 is inserted through the central bore of thepreform 30. Prior to insertion, the core strand 34 is coated first witha layer of semi-conductive material followed by a coating ofelectro-optic medium. The coated layers may be applied to the surface ofthe core strand via a variety of printing methods, including, but notlimited to, dip coating, electrodeposition, powder coating, spraycoating, or extrusion. Each layer is also preferably dried prior toinserting the coated core strand 34 into the preform 30.

Each of the conductive wires 33 a, 33 b and core strand 34 are attachedto the distal end 36 of the preform. This may be achieved, for example,by knotting the leading end of core strand and conductive wires afterinsertion through the length of the preform 30 or by attaching theleading ends to the distal end 36 of the preform 30 with an adhesive orfastener. After the core strand 34 and conductive wires 33 a, 33 b aresecured, the furnace 32 may heat the preform 30. The preform ispreferably made from a polymeric material, such as polycarbonate, thatwill be pliable and have a high ductility upon reaching an acceptabletemperature. Once heated, the distal end 36 of the preform 30 may bepulled using a draw tower or a fiber pulling system to draw down the thepreform 30. This causes a reduction in the dimensions of the preformwithin the middle drawn section 38 of the preform 30 including thediameter of the cavities extending axially through the preform, untilthe cross-section of the drawn section 38 has a cross-section similar tothe cross-section illustrated in FIG. 2 , for example. The ends ofmiddle drawn section 38 of the preform 30 may be cut to provide thecolor changing fiber, and finally the fiber walls may then be ablated,stripped, or cut to make connections to the conductive wires and corestrand electrodes.

By pulling a core strand having a pre-dried electro-optic medium coatingthrough a preform, extremely difficult post-drying steps previouslyrequired for removing water from hollow fibers filled with aqueousslurries, such as those described in U.S. Patent Application PublicationNo. 2018/0364518, are eliminated. The various embodiments of the presentinvention also allow for a high capsule fill factor (that is, fewcapsule void regions within the fiber). Furthermore, the use ofencapsulated electrophoretic media eliminates challenges associated withpigment settling, as well as allows one to cut these switchable fibersto any arbitrary length without losing all of the electro-optic mediumcontained within the fiber. In addition, if a central conductive wire isused as the core strand, one can create switchable fibers where thefiber has the same color on all sides, which could be beneficial whenweaving. By providing color changing fibers that display the same coloron all sides with circular cross section, the fibers may be weavedwithout the concerns associated with rectangular fibers related to theirtwisting leading to diminished color quality of the final woven fabric.

From the foregoing, it will be seen that the present invention canprovide color-changing fibers that can be integrated into textiles andother materials. The various embodiments of the invention allows one tomake fabrics that are inherently breathable and. flexible by weaving thecolor changing fibers. It is also possible to integrate these colorchanging fibers into a variety of other woven materials which can beused for interior design elements, architecture, wayfinding, etc. Thefibers according to the various embodiments of the present invention maybe used on standard looms and the manufacturing processes used toproduce the fibers are easily scalable. Furthermore, the fibers have thepotential to be independently addressed, and the electro-optic mediaapplied to each thread may contain different formulations. As a result,fabrics made using the color-changing fibers described herein may use aplurality of different fibers. For example, one set of fibers mayinclude an encapsulated electrophoretic media containing white and redpigments, a second set may include media containing white and greenpigments, and a third set may include white and blue pigments. Thefabric may be woven with the three sets of threads, such that the finalconfiguration of the weave would allow the combination of any of thefour colors in various switchable proportions and patterns to achieve awide spectrum of selectable colors for the fabric. The electrophoreticmedia is not limited to two pigments. The encapsulated electrophoreticmedia may alternatively include three or more pigments and/or a coloreddispersion fluid to allow for a potentially infinite number of opticalcombinations within the fabric, such as the electrophoretic mediadisclosed in U.S. Pat. No. 9,921,451. By using bistable electro-opticmedia, low power is required to switch the material and electroniccontrols used to switch the material may be detachable.

It will be apparent to those skilled in the art that numerous changesand modifications can be made in the specific embodiments of theinvention described above without departing from the scope of theinvention. Accordingly, the whole of the foregoing description is to beinterpreted in an illustrative and not in a limitative sense.

The contents of all of the aforementioned patents and applications areincorporated. by reference herein in their entireties.

1. A flexible color-changing fiber comprising: an annular wall and aconductive wire axially extending through the annular wall; a conductivecore strand surrounded by the annular wall and extending axially througha central portion of the fiber; and capsules including anelectrophoretic medium disposed on a surface of the conductive corestrand.
 2. The flexible color-changing fiber of claim 1, wherein theflexible color-changing fiber comprises a plurality of conductive wiresaxially extending through the annular wall.
 3. The flexiblecolor-changing fiber of claim 1, further comprising a layer ofsemi-conductive material between the conductive core strand and thecapsules including an electrophoretic medium.
 4. The flexiblecolor-changing fiber of claim 2, further comprising a layer ofsemi-conductive material between the capsules including anelectrophoretic medium and the conductive wire.
 5. The flexiblecolor-changing fiber of claim 1, wherein the electrophoretic mediumcomprises a non-polar solvent and two types of charged pigmentparticles, each type of charged pigment particle having a chargepolarity and color different from the other.
 6. A fabric comprising thecolor-changing fiber of claim 1.